Browse or search the archive via the Archive page. The channels list on the right show the channels available. Search via the form on the top left, or on a mobile device, opening the menu, as well as on the form on the archive pache.
Greyed out channels are archived channels in Slack. The messages themselves are readable and searchable, but the channels are disabled on Slack.
Log in with Slack to favourite messages, and download attachments directly from the website instead of opening Slack.